After the normal operation of the system for a period of time, the components of the reverse osmosis membrane may be contaminated by suspended solids or insoluble salts that may exist in the feedwater. The most common pollutants include calcium carbonate precipitation, calcium sulfate, barium sulfate, strontium sulfate precipitation, metal (iron, manganese, copper, nickel, aluminum, etc.) oxide precipitation, silicon deposition, inorganic or organic deposition mixtures, NOM natural organic matter, synthetic organic matter (such as scale inhibitors/dispersants, cationic polyelectrolytes), microorganisms (algae, mold, fungi), etc. The nature and speed of pollution depend on various factors, such as water quality and system recovery rate. Pollution usually develops gradually, and if not controlled early, it will damage membrane components in a relatively short period of time. When it is confirmed that the membrane element has been contaminated, or before long-term shutdown, or as a regular routine maintenance, it is recommended to clean the membrane element. When the reverse osmosis membrane system (or device) encounters the following problems, chemical cleaning or physical flushing is required: under normal water supply pressure conditions, the temperature corrected water production decreases by 10-15% compared to the normal value; To maintain normal water production, the water supply pressure has increased by 10-15% after temperature correction; The quality of the produced water has decreased by 10-15%, and the salt permeability has increased by 10-15%; The water supply pressure has increased by 10-15%; The pressure difference between each section of the system has significantly increased (there may be no instrument monitoring this parameter). Maintaining stable operating parameters mainly refers to water production flow rate, water production back pressure, recovery rate, temperature, and TDS. If these operating parameters fluctuate, it is recommended to check whether pollution has occurred or whether the actual operation of reverse osmosis is normal under the premise of changes in key operating parameters. The overall performance of the timed monitoring system is the basic method to confirm whether the membrane components have been contaminated. The impact of pollution on membrane components is gradual, and the degree of impact depends on the nature of the pollution. The cleaning cycle of the contaminated reverse osmosis membrane depends on the actual situation on site. The normal cleaning cycle is once every 3-12 months. When the membrane element is only slightly contaminated, it is important to clean the membrane element. Severe pollution can hinder the penetration of chemical agents into the contaminated layer, affecting the cleaning effect. What kind of pollutants to clean and how to clean them should be based on the on-site pollution situation. For complex situations where multiple pollutants coexist, the cleaning method is to use alternating low pH and high pH cleaning solutions (low pH should be used first, followed by high pH cleaning).
Pollution analysis 1. Calcium carbonate scale: Calcium carbonate scale is a mineral scale. When the scale inhibitor/dispersant addition system malfunctions, or when the acid pH adjustment system malfunctions and causes an increase in the pH of the feedwater, calcium carbonate scale may deposit. Early detection of calcium carbonate scale is essential to prevent crystal damage to membrane components caused by surface deposition. The early detection of calcium carbonate scale can be removed by reducing the pH value of the feedwater to 3-5 and running for 1-2 hours. For long-term sedimentation of calcium carbonate scale, low pH citric acid solution can be used for cleaning and removal. 2. Calcium sulfate, barium sulfate, strontium sulfate scale: Sulfate scale is a mineral scale that is much harder than calcium carbonate scale and is not easy to remove. Sulfate scale can deposit when the scale inhibitor/dispersant addition system malfunctions or when sulfuric acid is added to adjust pH. Early detection of sulfate scale is essential to prevent crystal damage to membrane components caused by surface deposition. Barium sulfate and strontium sulfate scales are difficult to remove because they are almost insoluble in all cleaning solutions, so special attention should be paid to prevent the formation of such scales. 3. Metal oxide/hydroxide pollution: Typical metal oxide and hydroxide pollution includes iron, zinc, manganese, copper, aluminum, etc. The formation of such scale may be caused by corrosion products of equipment pipelines, containers (tanks/tanks), or metal ions, chlorine, ozone, potassium, permanganate oxidized in the air, or by the use of iron or aluminum coagulants in pre-treatment filtration systems. 4. Polymeric silicon scale: the scale of silicon gel layer is caused by supersaturated state of soluble silicon and polymer, and it is very difficult to remove. It should be noted that this type of silicon contamination is different from the contamination of silicone gel material. Silicone contamination may be caused by association with metal hydroxides or organic compounds. The removal of silicon scale is very difficult, and traditional chemical cleaning methods can be used. The existing chemical cleaning agents, such as ammonium hydrogen fluoride, have been successfully used in some projects, but the operational hazards and damage to equipment of this method must be considered when using it, and protective measures must be taken. 5. Colloidal pollution: Colloids are inorganic substances or particles of organic and inorganic mixtures suspended in water, which do not precipitate due to their own gravity. Colloidal substances typically contain one or more of the following main components, such as iron, aluminum, silicon, sulfur, or organic compounds.
6. Non soluble natural organic matter pollution (NOM): Non soluble natural organic matter pollution (NOM) is usually caused by the decomposition of nutrients in surface water or deep well water. The chemical mechanism of organic pollution is complex, with the main organic components being humic acid or humic acid. Insoluble NOM adsorbed on the membrane surface can cause rapid pollution of RO membrane elements. Once absorption occurs, the pollution process of gradually forming gel or block will begin.
7. Microbial deposition: Organic sediment is generated by bacterial slime, fungi, molds, and other pollutants, which are difficult to remove, especially when the water supply pathway is completely blocked. Blockage in the water supply pathway can make it difficult for clean water to enter the membrane element evenly and fully. To suppress the further growth of this sediment, it is important not only to clean and maintain the RO system, but also to clean the pre-treatment, pipelines, and end heads.
